The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle as a function of various system and operator-initiated inputs.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing tractive power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. However, the vast majority of such part-time transfer cases include a mechanical "mode" shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions.
Alternatively, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. In one typical application, a viscous transmission device, such as a viscous coupling, is located in the driveline between the driven and non-driven wheels to provide "on-demand" four-wheel drive operation. Modernly, it is known to incorporate the "on-demand" feature into a transfer case by replacing the mechanical mode shift mechanism with a multi-disc clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated state such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque "on-demand" to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an "on-demand" power transfer system is disclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein a hydraulically-actuated clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function the wheel speed difference (i.e., the wheel slip) between the front and rear wheels.
While numerous variations of such hydraulically-actuated "on-demand" power transfer systems are known, they are primarily intended for road use in passenger cars and not for off-road recreational or sport/utility vehicles. In addition, such conventional "on-demand" transfer cases are single-mode power transfer mechanisms which do not provide means for permitting the vehicle operator to deliberately shift between traditional two-wheel drive and part-time four-wheel drive modes of operation. Thus, the need exists to develop a transfer case incorporating an "on-demand" four-wheel drive feature which can be effectively over-ridden upon the vehicle operator selectively shifting the transfer case into either of a two-wheel drive mode or a part-time four-wheel drive mode.